224 BELL SYSTEM TECHNICAL JOURNAL 



bulletins of the Station, and in a graduate thesis ^ of the junior author 

 and J. D. Corley. 



As early as 1920, the senior author proposed the use of electric wave 

 reflection in railway safety systems ^ and entertained the idea of 

 frequency-modulated transmission with beat-tone detection for 

 measuring distance along a track. Radio wave reflection for aircraft 

 altitude determination was considered at times from 1926 to 1930 

 when a patent application was filed '' for an arrangement similar to that 

 which has been worked out, including the use of a frequency meter to 

 give continuously a visual indication of the altitude. 



At that time, however, a really practical terrain clearance indicator 

 could not be built due in large part to the lack of suitable radio in- 

 strumentalities. Vacuum tubes capable of operating on frequencies 

 approximately fifty times higher than those generally available were 

 indicated as necessary before a satisfactory system could be built. 



A long-range program, however, of vacuum tube development for 

 high frequencies was under way in Bell Telephone Laboratories. This 

 resulted in the production of suitable tubes, and they were described 

 by A. L. Samuel ^ to the Institute of Radio Engineers in October, 

 1937. One of these was capable of providing a stable output of be- 

 tween five and ten watts at a frequency of approximately 500 mega- 

 cycles, so it became feasible to undertake the development of a practical 

 terrain clearance meter. 



The Japanese have been experimenting recently with apparatus 



operating upon the same basic theory and a paper ^ was published in 



Japanese in 1936. A later paper i" was published in English in 1938 



by the same author, which describes the apparatus and the results of 



tests made on the ground over short distances with the equipment at 



rest. 



Technical Problems 



At the time this development was undertaken a number of questions 

 presented themselves as to what the earth's surface would do to the 

 incident wave in reflecting it. It seemed possible that the signal 

 might be so scattered and broken in reflection by small irregularities 

 that the echo would be more like static than a useful signal. 



Even if the reflected signal proved satisfactory over the smoother 

 surfaces, it was hard to predict what would happen when flying over 

 timber land or over very irregular mountainous terrain. There was 

 also the question of what would happen when the surface happened 

 to be that of a city where an airplane flying at 250 to 300 feet per second 

 passes over several buildings and streets with abrupt altitude changes 

 of possibly hundreds of feet several times in the course of one second. 



